Spinal reconstructive surgery may be used to correct anterior/posterior sagittal plane (Z-axis) imbalances ranging from abnormal (e.g., 40 mm) to severe (e.g., 90 mm) and/or to correct lateral coronal plane (X-axis) imbalances (e.g., greater than 20 mm). During such reconstructive surgery, vertical spacings between adjacent vertebrae along the length of the spine (Y-axis) may be adjusted using intervertebral spacers, rods, plates and the like.
Current products are not sufficient to provide full three-dimensional spinal re-alignment. The quantitative data provided by conventional instruments concerns the angles, e.g., ISSG parameters such as LL, PI, SS, CSVL or linear measurements in other planes, such as SVL in the sagittal plane and CSVL in the coronal plane. Current techniques do not account for the Y-axis, and the definition of vertical stability along the Y-axis of the human spine has not been adequately established. Surgeons cannot rely on angles alone to restore three-dimensional spine alignment. Additional quantitative measures are needed to ensure neurological preservation and recovery by preventing nerve root stretch during distraction and nerve root impingement/buckling of dura during osteotomy, optimize lordotic/kyphotic expandable cage fit, and optimize neuro-foraminal volume. Other factors that need to be measured are the tension of the posterior longitudinal ligament (PLL) and axial height. However, currently available products do not measure these parameters.
A shortcoming of conventional spinal column measurement techniques along the Y-axis is that overall sagittal alignment guides may be met but intervertebral spacing between a particular set of vertebrae may be severely impaired. For example, a patient may have dramatic over-distraction of two adjacent vertebrae, yet pelvic incidents, pelvic tilt, sacral slope and sagittal vertical alignment may be within a normal range. As another example, extreme shortening may occur at a specific location along the Y-axis of the spine, e.g., caused by a vertebra that is located anterior to an adjacent vertebra, yet sagittal plane SVA, coronal plane CSVL and center sacral vertical line may be within a normal range.